Paul et al., U.S. Pat. Nos. 2,950,389 and 2,939,952, disclose the use of a three-dimensional quadrupole ion trap and the theory thereof. The use of an ion trap as an ion source of a mass spectrometer is known. Reference herein to an r.f. ion trap is understood to mean a quadrupole type apparatus following the teaching of Paul et al. The r.f. ion trap mass spectrometer enables molecules and/or ions of a source to be analyzed and detected. The ions are collected in an interaction region defined by a torroidal electrode, typically having a hyperboloid contour, and a pair of end cap electrodes, also usually including hyperbolic surfaces. An r.f. voltage and a DC voltage are applied to the torroidal electrode while a DC voltage pulse is applied to only one of the end cap electrodes. The other end cap electrode is at ground potential. Ions are formed in the interaction region by supplying to the region electrons that interact with the molecules.
The ions are accumulated in the trap interaction region to produce a mass spectrum with acceptable signal-to-noise ratio. The ions are also compressed in physical space and the velocity thereof is reduced by interaction with a neutral, buffer gas in the interaction region of the ion trap. Multiple collisions between trapped ions and the neutral, buffer gas reduce the energy of the ions and cause a smaller and cooler ion cloud to be localized in the ion trap central interaction region. The spatially compressed ion cloud having relatively low velocity ions is much better suited to mass analysis than an ion beam dispersed over a much larger volume.
In one prior art arrangement, the ions located in the trap are somewhat efficiently removed and transferred to form an ion beam which enters a reflection type mass spectrometer, of the type disclosed by Mamyrin et al., Soviet Physics-JETP, Vol. 37 (1973), pages 45-48, in an article entitled "The Mass-Reflectron, A New Non-Magnetic Time-of-Flight Mass Spectrometer with High Resolution."
In the prior art, the ions are either "sucked out" or "pulsed out," i.e., "pushed out" of the interaction region through an aperture in one of the end cap electrodes to form the ion beam. Bonner et al., International Journal of Mass Spectrometry and Ion Physics, Vol. 10 (1972/73), pages 197-203 in an article entitled "Ion-Molecule Reaction Studies with a Quadrupole Ion Storage Trap" discloses an ion trap in which positive ions are transferred from the ion trap to a quadrupole mass spectrometer analyzer using a pulse out mode wherein a negative voltage pulse is applied to a front end cap electrode having an opening therein through which the ion beam propagates. Fulford et al., Journal of Vacuum Space and Technology, Vol. 17 (1980), pages 829-835, in an article entitled "Radio-Frequency Mass Selective Excitation and Resonant Ejection of Ions in a Three-dimensional Quadrupole Ion Trap" discloses an apparatus similar to that disclosed by Bonner et al., but operated in the suck out mode, wherein a positive voltage pulse is applied to a back end cap electrode opposite from the front end cap electrode. Mather et al., International Journal of Mass Spectrometry and Ion Physics, Vol. 28 (1978), pages 347-364, in an article entitled "The Quadrupole Ion Storage Trap (QUISTOR) As a Low-Pressure Chemical Ionization Source for a Magnetic Sector Mass Spectrometer" discloses an ion trap operated in a pulse out mode as an ion source for a magnetic sector mass spectrometer.
More recently, ion traps have been used as integrating ion sources for time-of-flight mass spectrometers. Ions have been ejected from the ion trap into the time-of-flight mass spectrometers using one of the suck out mode or the pulse out mode, as disclosed by Chien et al., Rapid Communications in Mass Spectrometry, Vol. 7 (1993), pages 837-843, in an article entitled "Enhancement of Resolution in a Matrix-Assisted Laser Desorption Using an Ion-Trap Storage/Reflection Time of Flight Mass Spectrometer" and Fountain et al., Rapid Communications in Mass Spectrometry, Vol. 8 (1994), pages 487-494, in an article entitled "Mass-Selective Analysis of Ions in Time of Flight Mass Spectrometry Using an Ion Trap Storage Device."
In the prior art, an arrangement of gridded electrodes (not an r.f. ion trap) was employed to produce an electrostatic field configuration which enhanced the local density of positive ions trapped in an electron beam. Through a combination of static electric fields and suitable pulses applied to the electrodes, these ions were efficiently extracted into a time-of-flight mass spectrometer. This arrangement is described by M. H. Studier in The Review of Scientific Instruments, Vol. 34, No. 12, (December 1963), pages 1367-1370.
In ion trap time-of-flight mass spectrometers it is important for the ion beam, as it enters the time-of-flight mass spectrometer, to have all ions with different energies at the same spatial location at the same time as can be achieved by an ion beam having parallel trajectories for different energies, i.e., a collimated ion beam. This is particularly true of time-of-flight mass spectrometers including a reflector and detector, as disclosed by Mamyrin et al. (supra).
In our studies, we have found that the prior art pulse out, i.e., push out, and suck out modes do not produce collimated ion beams. Instead, the push out method produces a divergent ion beam which is only partially transmitted through the opening in the front end cap electrode of the ion trap. Hence, the efficiency in transferring ions from the ion trap to the time-of-flight mass spectrometer is materially reduced because there is a substantial decrease in the number of ions in the beam which is coupled to the spectrometer.
For the suck out case, we have found increasing the extraction voltage applied to the apertured front end cap causes greater divergence of the ion beam with resulting decreases in efficiency.
Our investigations have found the prior art suck out technique results in an ion beam having a strongly convergent trajectory, with a crossover point somewhat downstream of the front end cap electrode. While none of the ions in the ion beam strike the front end cap electrode, the resulting ion beam is so divergent downstream of the crossover point that the ion beam cannot be focused back into a parallel beam without strong ion optics which introduce dispersion in the ion flight times.
We have found that the electric fields resulting from the voltages applied to the end caps are curved significantly and have substantial gradients in the regions where the ion beam is formed in the ion trap, using both the suck out and push out methods. In addition, the electric field gradients are asymmetrical, with gradients occurring only on the side of the ion beam between the polarized end cap electrode and the point or region in the ion trap where the beam is initially formed. Hence, neither of the prior art suck out or pull out techniques is capable, by itself, of producing a parallel ion beam suitable for efficient transmission through a time-of-flight mass spectrometer of the reflection type disclosed by Mamyrin et al.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for forming a collimated ion beam.
Another object of the present invention is to provide a new and improved ion trap for forming a parallel ion beam, particularly useful for a time-of-flight mass spectrometer, wherein virtually all of the ions with different energies enter the spectrometer at substantially the same time.
Another object of the invention is to provide a new and improved method of and apparatus for forming a collimated ion beam with an ion trap having a torroidal electrode excited by an r.f. source and a pair of end cap electrodes, one of which includes an opening through which the collimated ion beam flows, wherein the ion trap is constructed in such a manner that the ion beam is not incident on the apertured end cap.
Another object of the present invention is to provide a new and improved method of and apparatus for forming a parallel ion beam through the use of an ion trap including a torroidal r.f. excited electrode and a pair of end cap electrodes, one of which is apertured, wherein the ion beam is formed so that it does not converge on a point downstream of the apertured end cap and then diverge.
An additional object of the invention is to provide a new and improved ion trap method and apparatus, wherein the location in the ion trap where the ion beam is initially formed lies along a relatively straight electric field line having virtually no curvature in the ion trap.